This invention relates in general to a photoresponsive array of semiconductor elements for electronically detecting or reading light contrasting images provided by a medium such as a document. More specifically, the present invention is directed to improving the performance of a high density photodiode array of the thin film type that is placed between a document to be read or scanned by the array and a light source for illuminating the document.
Light from the light source passes through transparent window areas in the array and impinges on the document which is in close proximity to or in contact with the array. The reflected light from the document illuminates the photodiodes of the array, which are then electronically interrogated or read during a scan to provide a signal indicative of the images on the document. The spatial density of the photodiodes determines the optical resolution of the array, each diode constituting a pixel element. The array can be in the form of a line scanner or a full page scanner.
Such arrays are generally known in the art as thin film backlit arrays which rely on proximity focusing, i.e., the reflected light from the document being read impinges on the photodiodes of the array without passing through any focusing elements such as collimators, lenses or the like.
U.S. Pat. Nos. 4,149,197 and 4,660,095 illustrate thin film, backlit arrays of the proximity focusing type wherein the respective inventors teach that the photoresponsive elements or light sensors of their respective arrays must be shielded from direct impingement by light from the light source in order to avoid light saturation of the associated light sensors.